1. Field of the Invention
The present invention relates to a device for driving in a reciprocating manner an optical unit used in a copying machine, a manuscript reading apparatus or the like.
2. Description of the Prior Art
In a copying machine, a manuscript reading apparatus or the like, wherein a slit exposure operation is carried out, an optical unit having a light source for illuminating a manuscript is reciprocated to scanningly illuminate the manuscript supported on a transparent manuscript carrying plate and to guide light reflected by the manuscript to a photoreceptor, such as a sensitive member or CCD, through a lens device.
Such optical unit is accelerated from an appointed position called a starting or home position and then is moved at a constant speed in order to scan the manuscript. The unit then is braked and stopped after completion of scanning of the manuscript (scanning movement or travel). The optical unit then is accelerated in a direction opposite to the direction of scanning travel, and then is braked when returned to the starting or home position after the completion of such accelerated travel (return movement or travel).
However, it is necessary during scanning travel that the speed of the unit does not fluctuate within a range of travel during which the manuscript is scanned. In addition, it is necessary during return travel that such movement is conducted speedily in order to minimize operator waiting time. In the past, a single pulse motor has been used as a driving source to achieve this reciprocating movement. This single pulse motor has been operated to rotate in opposite directions and to achieve both constant speed travel or accelerated travel in both directions. However, when scanning travel and return travel are carried out by a single pulse motor, a difference exists between the speed of scanning travel and the speed of return travel. If characteristics of the pulse motor are set to avoid noise during return travel, then noise is increased during scanning travel, then noise is increased during scanning travel. Also, if characteristics of the pulse motor are set to avoid noise during scanning travel, then noise is increased during return travel.